Conventional systems for delivering air for the heating and cooling of buildings use one of three different techniques. A constant volume system continuously supplies a constant volume of air and varies the temperature of the air that is being supplied in order to achieve a temperature change in the space. Variable volume systems operate under simple on/off control or use analog throttling damper or fan modulation to vary the flow rate.
All of these conventional systems have serious shortcomings. A typical constant volume system uses a thermostat in the space that senses the ambient temperature and sends a feedback signal. If the air temperature is above the set point temperature, the air supply temperature is reduced. Conversely, the air supply temperature is increased if the sensed temperature is below the set point. Although constant volume systems are relatively simple and provide good ventilation, they have suffered a decline in popularity due primarily to their energy inefficiency. The problem is that when the load is low, a constant volume system delivers more air than is necessary to maintain the set point temperature. This results in a waste of fan energy which takes on increasingly adverse significance as energy costs increase.
Variable volume on/off systems are widely used because they are simple, economical to install and relatively inexpensive to operate. However, there are important disadvantages in that there is no ventilation during off cycles, the temperature in the space is non-uniform, there is considerable noise variation between on and off cycles, there is by necessity a significant dead band in the thermostat control, and they are not practical for use other than in single zone systems.
Variable volume systems that vary the flow using variable dampers or variable fans are advantageous in that they are able to closely track the load in the space and are efficient in fan energy use. However, they are also characterized by relatively high costs and complexity, noise variation caused by flow modulation, ineffective ventilation, and inadequate mixing at low air volumes and load.
Analog modulation techniques for varying the airflow are particularly disadvantageous when the air quantity is reduced under conditions of low loading. When the flow if reduced, there is also a reduction in the air momentum, velocity, air throw, air mixing and air induction. This results in poor comfort to the occupants of the space and a compromise in the thermal efficiencies of the system. These problems have been addressed by using air terminals in which the discharge area is restricted to maintain a relatively constant velocity as the flow rate is reduced. However, there is still a reduction of mass in the discharge air and associated limitations in the kinetic energy, momentum, mixing, induction and air throw. At low supply pressure, these problems are especially pronounced. For all of these reasons, the so-called constant velocity, variable area devices are deficient as to the range of loading conditions they can successfully handle.
Response rates have been another problem associated with variable damper mechanisms. Standard practice is to provide a slow opening and closing time for the damper in order to better match the dynamic response of the space to the response of the controls, the sensing elements and the damper mechanism. If the response is too rapid, unstable control of the damper can result and cause a “hunting” condition in which the damper is repeatedly repositioned without producing the correct air quantity. Conversely, if the damper opens and closes too slowly, the control of the temperature in the space suffers. This condition is referred to as “drift” and often results from efforts at avoiding the hunting effect at the expense of transient response. Reaching a compromise where the system is well tuned is always challenging and often labor intensive even if successful.
A further problem with prior art dampers is that they are subject to noise that results mainly when the air velocity changes. Air flowing through small areas at low flow rates can cause vibration of the hardware components and can also result in objectionable noise from the air itself. The result is that noise at objectionable levels can be produced, with varying noise at different flow rates making the situation even less acceptable.
Treating air in other ways such as for high or low humidity, oxygen depletion, or excessive carbon dioxide is subject to the same problems.